Equipment for measuring microcurrents and a circuit with microcurrent output sensors, as well as other equipment for handling microcurrents often has an electrical transmission line that has been air-wired in order to prevent contamination by outside current or leakage current generated when the microcurrent is transmitted (refer to JP (Kokai) 8[1996]-335,754). A guard pattern of the same potential as the air-wired electrical transmission line is generally made around the transmission line in order to prevent direct-current leakage current from flowing around the transmission line and to prevent charge current from flowing to the floating capacitance formed around the transmission line.
FIG. 4 is a typical example of an electrical transmission line 50 with a guard pattern 30. Guard pattern (conductive region) 30 is formed parallel to an electrical wire 20 that transmits microcurrents on a substrate 40. A plurality of studs 10 are disposed in guard pattern 30 along electrical wire 20. Electrical wire 20 and guard pattern 30 are separated by supporting the electrical wire 20 using insulation studs 10.
An example of a typical insulation stud 10 is shown in FIG. 3. Insulation stud 10 is a cylindrical insulator 12 made from Teflon (registered trademark) with a top electrode 11 and a bottom electrode 13 at either end. Electrical wire 20 is anchored by soldering it onto electrode 11. Moreover, insulation stud 10 is anchored to guard pattern 30 by joining guard pattern 30 and electrode 13 by soldering.
However, the temperature around electrical transmission line 50 changes over time. Because of this, the surface area contacting the atmosphere and the heat capacity differ between top electrode 11 and bottom electrode 13 connected to guard pattern 30; therefore, the rate of change in temperature at the two electrodes is not the same. As a result, a temperature difference is produced between the two electrodes while the peripheral temperature changes. When this occurs, a thermally stimulated current is produced in accordance with the temperature difference of insulator 12 and this current flows into electrical wire 20. In general, this thermally stimulated current is a very small microcurrent (usually on the order of several femtoamperes to several hundred femtoamperes), but the fact of thermally stimulated current cannot be disregarded when the current flowing to electrical wire 20 is a microcurrent on the same order as the thermally stimulated current or when the transmitted current must be measured at the same resolution as the thermally stimulated current.
There are methods whereby electrical transmission line 50 is closed in order to eliminate as much as possible the effects of peripheral temperature changes and thereby to control the thermally stimulated current. However, when the transmission line is closed, the effect of internal heat generation increases and there is an increase in the possibility of current leakage, and similar effects occurring due to the presence of humidity trapped inside the closed area. Therefore, it is preferred that the difference in the amount of temperature change between top electrode 11 and bottom electrode 13 be reduced without closing the electrical transmission line. Bottom electrode 13 and guard pattern 30 can be thermally separated in order to accomplish this, but when bottom electrode 13 and guard pattern 30 are completely electrically separated, bottom electrode 13 enters a state where it is said to be floating electrically and it becomes impossible to prevent direct-current leakage current from floating around the transmission line or to prevent the charge current from flowing to floating capacitance produced around the line because the line is not completely guarded. Therefore, it is preferred that bottom electrode 13 and guard pattern 30 be electrically connected while preventing heat conduction between the two.